This invention relates generally to nuclear reactors and, more particularly, to an apparatus and method for repairing or replacing welds in piping within a reactor pressure vessel of a reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A core shroud typically surrounds the reactor core and is contained within the RPV.
Boiling water reactors generally include piping for core spray cooling water. Core spray piping is used to deliver water from outside the RPV to core spray spargers inside the RPV. The core spray piping generally includes vertical downcomer pipes. Upper and lower sections of each downcomer pipe are typically joined together during reactor assembly. The upper piping section is trimmed to provide the proper length and fit-up with the lower piping section. The slip coupling welded to the upper and lower piping sections prevents coolant leakage and also provides the necessary structural strength needed in the downcomer piping system.
The core spray piping systems in operating BWRs are of welded construction. Welds in the spray piping are susceptible to stress corrosion cracking, particularly intergranular stress corrosion cracking (IGSCC). Reactor components are subject to a variety of stresses associated with differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other stress sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. Water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to stress corrosion cracking. Accordingly, there is a long felt need for devices and methods for repairing welds in downcomer pipes.